Transdermal (ie, through the skin) delivery of therapeutic agents affords a comfortable, convenient and noninvasive technique for administering drugs. The method provides several advantages over conventional modes of drug delivery. For example, variable rates of absorption and (eg, hepatic) metabolism encountered in oral treatment are avoided, and other inherent inconveniences--eg, gastrointestinal irritation and the like--are eliminated. Transdermal delivery also allows a high degree of control over blood concentrations of a particular drug and is an especially attractive administration route for drugs with narrow therapeutic indexes, short half-lives and potent activities.
Transdermal delivery can be either passive or active. Many drugs are not suitable for passive transdermal drug delivery because of their size, ionic charge characteristics and hydrophobicity. One method of overcoming this limitation is the use of low levels of electric current to actively transport drugs into the body through intact skin. This technique is known as "electrotransport" or "iontophoretic" drug delivery. The technique provides a more controllable process than passive transdermal drug delivery since the amplitude, timing and polarity of the applied electric current is easily regulated using standard electrical components. In this regard, electrotransport drug flux can be from 50% to several orders of magnitude greater than passive transdermal flux of the same drug.
Electrotransport devices generally employ at least two electrodes. Both of these electrodes are positioned in intimate electrical contact with some portion of the skin of the body. One electrode, called the active or donor electrode, is the electrode from which the therapeutic agent is delivered into the body. The other electrode, called the counter or return electrode, serves to close the electrical circuit through the body. In conjunction with the patient's skin, the circuit is completed by connection of the electrodes to a source of electrical energy, eg, a battery, and usually to circuitry capable of controlling current passing through the device.
Depending upon the electrical charge of the species to be delivered transdermally, either the anode or cathode may be the active or donor electrode. In this regard, if the ionic substance to be driven into the body is positively charged, then the positive electrode (the anode) will be the active electrode and the negative electrode (the cathode) will serve as the counter electrode, completing the circuit. On the other hand, if the ionic substance to be delivered is negatively charged, then the cathodic electrode will be the active electrode and the anodic electrode will be the counter electrode. Alternatively, both the anode and the cathode may be used to deliver drugs of appropriate charge into the body. In this case, both electrodes are considered to be active or donor electrodes. In other words, the anodic electrode can deliver positively charged agents into the body while the cathodic electrode can deliver negatively charged agents into the body.
Existing electrotransport devices additionally require a reservoir or source of the therapeutic agent that is to be delivered into the body. Such drug reservoirs are connected to the anode or the cathode of the electrotransport device to provide a fixed or renewable source of one or more desired species or agents. Examples of reservoirs and sources include a pouch as described in U.S. Pat. No. 4,250,878 to Jacobsen; a pre-formed gel body as disclosed in U.S. Pat. No. 4,382,529 to Webster; and a glass or plastic container holding a liquid solution of the drug, as disclosed in the figures of U.S. Pat. No. 4,722,726 to Sanderson et al.
Of particular interest herein is the transdermal electrotransport delivery of peptides, polypeptides, and proteins because of the problems encountered with more common drug administration routes such as oral delivery. Polypeptide and protein molecules are highly susceptible to degradation by proteolytic enzymes in the gastrointestinal tract and are subjected to an extensive hepatic metabolism when taken orally. Polypeptides and proteins usually require parenteral administration to achieve therapeutic levels in the patient's blood. The most conventional parenteral administration techniques are hypodermic injections and intravenous administration. Polypeptides and proteins are, however, inherently short acting in their biological activity, requiring frequent injections, often several times a day, to maintain the therapeutically effective levels needed. Patients frequently find this treatment regimen to be inconvenient, painful and with an attendant risk of, eg, infection.
Much effort has been expended to find other routes (other than parenteral injections) for effective administration of pharmaceutical polypeptides and proteins. Administration routes with fewer side effects as well as better patient compliance have been of particular interest. Such alternative routes have generally included "shielded" oral administration wherein the polypeptide/protein is released from a capsule or other container after passing through the low pH environment of the stomach, delivery through the mucosal tissues, eg, the mucosal tissues of the lung with inhalers or the nasal mucosal tissues with nasal sprays, and implantable pumps. Unfortunately, these alternative routes of polypeptide/protein delivery have met with only limited success.
Transdermal electrotransport delivery of polypeptides and proteins has also encountered technical difficulties. For example, water is the preferred liquid solvent for forming the solution of the drug being delivered by electrotransport due to its excellent biocompatability. The skin contains proteolytic enzymes which may degrade the polypeptide/protein as it is delivered transdermally. In addition, certain polypeptides/proteins, particularly those that are not native to the animal being treated, may cause skin reactions, eg, sensitization or irritation.
A number of investigators have disclosed electrotransport delivery of polypeptides and proteins. An early study by R. Burnette et al. J. Pharm. Sci. (1986) 75:738, involved in vitro skin permeation of thyrotropin releasing hormone, a small tripeptide molecule. The electrotransport flux was found to be higher than passive diffusional flux. Chien et al. J. Pharm. Sci. (1988) 78: 376, in both in vitro and in vivo studies, showed that transdermal delivery of vasopressin and insulin via electrotransport was possible. See, also, Maulding et al., U.S. Statutory Invention Registration No. H1160, which discloses electrotransport delivery of calcitonin in minipigs.
Several approaches (other than simply increasing the applied levels of electrotransport current) have been used to enhance transdermal electrotransport flux of polypeptide and protein drugs. One approach involves the use of flux enhancers such as ionic surfactants. See, eg, U.S. Pat. No. 4,722,726 to Sanderson et al. Another approach uses cosolvents other than just water to enhance electrotransport flux. See, eg, European Patent Application 278,473. Yet another approach involves mechanically disrupting the outer layer (ie, the stratum corneum) of the skin prior to electrotransport delivery therethrough. See, eg, U.S. Pat. No. 5,250,023 to Lee et al.
Further approaches to enhancing transdermal electrotransport drug flux involve creating a prodrug or an analog of the drug of interest and electrotransporting the prodrug or modified analog. For example, WO 92/12999 discloses delivery of insulin as an insulin analog having a reduced tendency to self-associate (apparently associated forms of insulin present in conventional pharmaceutical compositions reduce transdermal delivery of the insulin). The analogs are created by substituting aspartic acid (Asp) or glutamic acid (Glu) for other amino acid residues at selected positions along the insulin polypeptide chain. WO 93/25197 discloses delivery of both peptide and non-peptide drugs as pharmaceutical agent-modifier complexes or prodrugs wherein a chemical modifier (eg, a charged moiety) is covalently bonded to the parent pharmaceutical agent. The covalent bond is broken after the agent is delivered into the body, thereby releasing the parent agent.
Despite the above approaches, some polypeptides still exhibit poor transdermal electrotransport flux. In particular, peptide hydrophobicity is known to negatively impact electrotransport flux in vitro. Various parameters contribute to hydrophobicity, including the primary structure of a protein, ie, the amino acid sequence of the molecule, as well as the secondary structure of the protein, namely, the regular, recurring arrangement of the polypeptide chain along three dimensions. Such conformation can take the form of helical structures, such as an .alpha.-helix, or a more extended, zigzag conformation, known as the .beta.-conformation.
The .alpha.-helix has approximately 3.6 residues per turn of the helix. The R groups of the amino acids extend outward from the helix and intrachain hydrogen bonds are formed between the backbone carbonyl oxygen of each residue and backbone hydrogen atom attached to the electronegative nitrogen of the fourth residue along the chain. The basic unit of the .beta.-conformation is the .beta.-strand which exists as a less tightly coiled helix, with 2.0 residues per turn. The .beta.-strand conformation is only stable when incorporated into a .beta.-sheet, where hydrogen bonds with close to optimal geometry are formed between the peptide groups on adjacent .beta.-strands; the dipole moments of the strands are also aligned favorably. Side chains from adjacent residues of the same strand protrude from opposite sides of the sheet and do not interact with each other, but have significant interactions with their backbone and with the side chains of neighboring strands. For a general description of .alpha.-helices and .beta.-sheets, see, eg, T. E. Creighton, Proteins: Structures and Molecular Properties (W.H. Freeman and Company, 1993); and A. L. Lehninger, Biochemistry (Worth Publishers, Inc., 1975)
The Zimm-Bragg parameters, s and .sigma. (B. H. Zimm and J. K. Bragg J. Chem. Phys. (1959) 31:526-535), and Lifson-Roig equations (S. Lifson and A. Roig J. Chem. Phys. (1961) 34:1963-1974) are conventionally used to determine the stability of a helical segment in a given polypeptide. S represents the helix-coil stability constant and .sigma. is the nucleation factor. Based on these parameters, the likelihood of certain regions of polypeptide molecules to form .alpha.-helices and .beta.-sheets can be predicted using various calculations and computer programs. See, eg, Finkelstein, A. V. Program "ALB" for protein and polypeptide secondary structure calculation and prediction (1983). Deposited at the Brookhaven Protein Data Bank, Upton, N.Y. and at the EMBL, Heidelberg, Germany; Finkelstein, A. V. Biopolymers (1977) 16:525-529; Finkelstein et al. Proteins: Structure, Function and Genetics (1991) 10:287-299. Furthermore, Chou-Fasman probabilities (Chou, P. Y. and Fasman, G. D. Ann. Rev. Biochem. (1978) 47:251-276) have been used to assess the propensity of a particular amino acid residue to favor or disfavor .alpha.-helical and .beta.-sheet formation.
However, these principles have not been previously applied to alter the hydrophobic properties of a given polypeptide in order to increase the electrotransport flux thereof.